1. Field of the Invention
The invention relates to an improved process and apparatus for the cleavage of alkylaryl hydroperoxides. The invention is particularly useful for the acid-catalyzed cleavage of cumene hydroperoxide (CHP) to give phenol and acetone.
2. Discussion of the Background
The process of acid-catalyzed cleavage of cumene hydroperoxide to phenol and acetone has been of particular industrial importance for a long time and continues to be so today. In the preparation of phenol from cumene by the Hock process, cumene is oxidized in a first reaction step, known as oxidation, to form cumene hydroperoxide (CHP) and the CHP is subsequently concentrated to from 65 to 90% by weight in a vacuum distillation step known as concentration. In a second reaction step, known as cleavage, the CHP is cleaved into phenol and acetore by action of an acid, usually sulfuric acid. In this step, dimethyl phenyl carbinol (DMPC), formed as a side product of the oxidation, is cleaved into xcex1-methylstyrene (AMS) and water in an equilibrium reaction. DMPC also reacts with CHP to form dicumyl peroxide (DCP). Unreacted DMCP remains in the cleavage product. After neutralization of the cleavage product, the mixture is usually worked up by distillation.
In the cleavage reaction, high boilers (dimers, cumylphenols, bisphenols) are formed from AMS and DMPC and are later separated as residue in the distillation. The AMS present after the neutralization is hydrogenated to cumene during distillation and returned for oxidation. Unreacted DMPC remaining after cleavage ends up as either a high boiler in the residue, or reacts further in the hot phenol columns to form AMS from which high-boiling secondary components are in turn formed. DCP is stable at customary cleavage temperatures (from 50 to 70xc2x0 C.). It can decompose thermally in the hot phenol columns to form o-cresols. On the other hand, DCP can be cleaved at temperatures above 80xc2x0 C. in the presence of acid into phenol, acetone and AMS. It is therefore advantageous to achieve complete reaction of the remaining DMPC and the DCP by selectively increasing the temperature in the presence of the acid catalyst in the cleavage. This converts most of the DMPC into AMS and converts DCP virtually completely into phenol, acetone and AMS.
3. Description of the Related Art
Thermal post-treatment of the cleavage product has been described in U.S. Pat. No. 2,757,209, in which temperatures above 100xc2x0 C., especially in the range from 110 to 120xc2x0 C., were used. The objective of this thermal post-treatment was complete dehydration of the DMPC to AMS. U.S. Pat. No. 4,358,618, on the other hand, describes a thermal post-treatment with the objective of completely converting the DCP formed in the cleavage into phenol, acetone and AMS, by employing temperatures of from 120 to 150xc2x0 C. U.S. Pat. No. 5,254,751 describes thermal post-treatment with the same objective as that of U.S. Pat. No. 4,358,618, but with temperatures in the range from 80 to 110xc2x0 C. Finally, post-treatment is carried out above 150xc2x0 C. in DE 197 55 026 A1. Thus drastically different optimum temperature ranges for the thermal post-treatment of the cleavage product from the production of phenol have been described.
In all the above-mentioned thermal post-treatment processes, the cleavage product is first heated by means of steam in heat exchangers and, after a sufficient reaction time, is then cooled by means of water in heat exchangers. Depending on the temperature chosen for the thermal post-treatment, this results in specific steam consumption of about 0.2 metric tons of steam per metric ton of phenol. It has been found that temperatures above 100xc2x0 C., especially temperatures above 120xc2x0 C., result in increased deposition (fouling) of high-boiling by-products in the heat exchangers. This deposition is associated with a drastic impairment of heat transfer. Especially in the apparatuses for heating the product by means of steam, organic deposits formed on the hot heat exchange surfaces on the product side require these apparatuses to be cleaned at relatively short intervals, such as a few weeks. As the temperature increases, the degree of fouling also increases.
Heating the cleavage product by means of a recuperator in which the cleavage product entering the thermal post-treatment is preheated by the cleavage product leaving the thermal post-treatment enables the steam consumption to be reduced. The problem of fouling is not prevented by this method, as is described in U.S. Pat. No. 6,057,483.
DE 100 21 482 proposed a solution to this problem. In this process for the thermal post-treatment of the cleavage product resulting from the acid-catalyzed cleavage of cumene hydroperoxide into phenol and acetone, the cleavage product to be thermally treated is heated in a reactor using the heat of reaction of at least one exothermic reaction occurring in this reactor. This achieves high selectivity in the post-treatment while at the same time lowering energy costs and achieving a higher operating period of the heat exchangers by avoidance of fouling.
A disadvantage of this process, in which the heat of reaction liberated in the cleavage of CHP is normally used, is the fact that in order to obtain high residual CHP contents at the outlet of the cleavage product and thus in the circulated stream, it is necessary to employ high circulating flows so as to be able to dilute the concentrate entering the cleavage to a sufficient extent. Although a second introduction of CHP concentrate is more advantageous from this point of view, as known to those skilled in the art, it requires additional safety equipment and is therefore very costly.